In typical boiling water reactors used for power generating operations, reactor coolant is circulated endlessly cycling through a flow path comprised of a core entrance plenum located below the heat producing fuel core, up through the fuel core itself, on through an upper plenum region located above the core and which serves to collect all the coolant passing through fuel assemblies comprising the core, then on upward through an area for separation of steam from liquid water overhead of the core upper plenum, and finally around back downward outside the core, along a region termed the downcomer annulus, to return to the core lower plenum. If the reactor is designed as a natural circulation type boiling water reactor, this final flow path outside the core is directed and uninterrupted. A cylindrical member enshrouding the core and extending some distance both above and below the elevations containing the fuel core is positioned between upwardly flowing coolant passing through the reactor core, and downwardly flowing coolant recycling back to the core lower plenum. If the reactor type is a forced circulation reactor, some form of pumping mechanism is positioned outboard of the core shroud member along this portion of the flow path to amplify the pressure head otherwise present in the core lower plenum region.
The water coolant in such boiling water reactors during their power generating operation exists, at the core entrance, in the form of a subcooled liquid. This subcooled liquid has been produced by mixing, early along the downcomer annulus flow path, two streams: a feedwater steam that has large temperature subcooling relative to reactor operating pressure conditions, and a saturated liquid stream which has been derived by the partitioning of two-phase steam-water mixture produced downstream from the exit from the core. The feedwater stream has a mass flow rate that is controlled to match the reactor steam output mass flow rate, so that the coolant inventory and water level within the reactor remain nominally constant. The aforementioned partitioned saturated liquid stream typically has a mass flow rate many times the mass flow rate of the feedwater stream, so that the temperature of the mixed stream arriving in the core lower plenum lies closer to the coolant saturated conditions than to the feedwater entrance conditions.
As the reactor coolant passes through the core, heat is transferred from the fuel assemblies to the coolant. The water coolant emerges from the heat producing fuel core as a two-phase mixture of steam vapor and liquid water, the proportions of which vary depending on such factors as the power output from the fuel, the amount of subcooling present in the feedwater, the total hydrodynamic flow resistance presented by the fuel core design and structure and its wetted surface, and the amount of orificing representing restrictions to flow immediately prior to the entrance of the coolant into the individual core fuel assemblies.
Conventional fuel assemblies of boiling water reactors are composed of a multiplicity of fuel units, such as rods, grouped together in bundles, with each bundle surrounded by an open ended channel for flow lengthwise therethrough of coolant water. These channeled bundles of fuel units are spaced apart from each other to provide intermediate spaces for insertion of control blades. Thus, there are ample areas for coolant water bypass flow beyond close proximity to the heat producing fuel units within a bundle.
Bypass flow coolant water passes through the fuel core without closely encountering the high energy emanating from the fuel and enters the core upper plenum consisting substantially of saturated liquid with perhaps a small amount of steam. This bypass effluent joins the two-phase steam-water mixtures exiting from individual fuel assemblies comprising the core. These two effluents rapidly mix together within the core upper plenum losing identity from their origin, with the result of a combined overall steam-water mixture containing significant proportions of water.
The circulating coolant emerging upward from the fuel core surrounded by the core shroud as a two-phase mixture of vaporized steam and liquid water varies in proportions which are dependent upon several factors, including power output of the fuel, the amount of subcooling present in the coolant passing up into the fuel core, the total hydrodynamic flow resistance presented by the fuel geometry and wetted surfaces, and the amount of orificing representing restrictions to fluid flow just prior to the coolant's entry into the fuel core assembly.
The two-phase mixture comprising vaporized steam bubbles entrained within liquid water coolant produced within the heat generating fuel core, upon reaching the upper region of the above core plenum adjacent the shroud top, goes through a buoyancy-driven classification of the steam vapor and liquid water into separate components. Steam, being the lighter component and present in the form of gas bubbles entrained within a continuous saturate liquid water medium, is carried by buoyancy forces to the mixture free-surface. The steam bubbles emerge, or break out from the free-surface while the liquid water, being the heavier component of the mixture, remains in place except for a short steam layer containing dense moisture produced immediately above the free-surface by the breaking out action of the steam bubbles.
The thus classified water remaining within the mixture is continuously displaced by the circulating coolant comprising a steam-water mixture emerging from the fuel core and flowing up within the shroud, whereupon the liquid coolant water becomes diverted to a lateral outward direction of flow over the top rim or edge of the core shroud. As this deflected water travels outward and over the top rim of the core shroud, steam bubbles floating upward towards the mixture free-surface of the circulating coolant are swept along by the laterally moving liquid water coolant. The rising steam bubbles in the peripheral region of the shroud are more susceptible to being enveloped and carried along by the lateral flow. Moreover, since gaseous steam bubbles typically comprise about 80 percent of the volume of the upward flow within the upper plenum area from the fuel core, the quantity of diverted steam can be significant. Also, the lateral flow velocity of the liquid coolant component increases with the distance extending outward from the center of the core shroud. Thus, the highest lateral velocities developed by the liquid coolant occur at the periphery of the shroud.
Established potential flow theory holds that at progressively closer vertical positions from the mixture free-surface straight downward to the top rim of the shroud, water lateral flow velocities will be highest close by the shroud upper rim and the water velocities will diminish at successively higher elevations along this same diameter. Under these conditions the now fast-moving lateral flowing water component collides with the steam-water mixture rising upwardly through the shroud, whereby some steam bubbles of the mixture are swept laterally outward over the top rim of the shroud and carried along with the circulating coolant flow downward into the downcomer annulus flow path external to the core shroud.
Steam carried over into the downcomer annulus with the circulating coolant rapidly becomes condensed when combined with the incoming makeup coolant feedwater. Since steam has a very high thermal energy per unit mass, even a small amount of steam carryover such as, for example, only 1.7 percent by weight steam carryover experienced by the Dodewaard natural circulation reactor, substantially reduces the effective cooling of the total combined circulating coolant. When this "steam heated" coolant enters into the fuel core, its inadvertently added thermal energy results in reduced cooling effectiveness whereby boiling with the production of vaporized steam bubbles begins closer to the inflow of the coolant into the fuel core, namely at a lower location of the fuel core with an upward circulating coolant.
This steam carryover heating of the coolant entering the fuel core causes several troublesome conditions in the reactor performance. For example, as the occurrence of voids increases within the fuel core due to premature boiling and formation of steam bubbles, higher irreversible pressure drops occur within the core. This in turn impedes the recirculation flow. Also, coolant higher void content produces greater negative reactivity due to reduced neutron moderation whereby control rods must be further withdrawn from the fuel core which diminishes the length of fuel burnups for a given core initial enrichment, among other adverse effects.